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What You Need to Know
About Mold
Research Report - 0208
January-2002
Joseph Lstiburek, Terry Brennan and Nathan Yost
Abstract:
This article answers your questions about mold, what it is, where it grows, how it
spreads, how can I prevent it.
What You Need To Know
About Mold
Nathan Yost, MD
Joseph Lstiburek, Ph.D, P.Eng.
Terry Brennan, MS
What Is Mold?
Mold, one type of fungus, is different from plants,
animals and bacteria. Molds are decomposers of dead organic
material such as leaves, wood and plants. Molds sometimes
can infect living plants and animals. The spores and hair-like
bodies of individual mold colonies are too small for us to see
without a microscope. When a lot of mold is growing on a
surface, it often appears black or green. The color of mold is
influenced by the nutrient source and the age of the colony. If
mold is growing behind vinyl wallpaper, colorful pink or
purple splotches may appear. Mold growing on fabric is
called mildew.
What Does Mold Need to Grow?
Mold needs water to grow; without water mold cannot
grow. Mold also needs food, oxygen and a temperature
between 40 degrees and 100 degrees F. Since mold decomposes dead organic (once living) material it can grow on
wood, the paper facing on gypsum board (drywall) and other
materials made from wood. Molds secrete digestive fluids
that decompose the substrate, making nutrients available.
Mold can also digest some synthetic materials such as
adhesives, pastes and paints. While mold cannot get nutrients
from inorganic material such as concrete, glass and metal, it
can grow on the dirt present on these surfaces. Molds prefer
damp or wet material. Some molds can get moisture from the
air when the air is very damp, that is when the relative
humidity is above 80%. The high humidity makes surfaces
damp enough for mold to grow.
How Does Mold Spread?
Mold can grow by extension of hyphae that are like tiny
root hairs. In this way, a small colony of mold can expand to
cover many square feet of material. Mold can also make
spores that are like very small seeds. Spores can survive
conditions that are too sunny, hot, cold, dry or wet for mold to
grow. When spores are released they can be carried by air or
water to new locations. Some spores are so small that they are
more affected by air currents than by gravity. When spores
land on a damp surface that has food and oxygen available,
and if the temperature suits them, they will start to grow. It is
important to realize that mold spores are present everywhere,
in outside air as well as indoor air – unless very special
precautions are taken to remove or kill them.
Wherever there is decaying organic material (leaves,
mulch, wood) mold and mold spores are also present.
Everyday we are exposed to airborne mold spores from
outdoor sources, sometimes at high concentrations. It is
almost impossible to create a mold free space or to keep a
space mold free. What we can do – and should do – is control
the amount of mold in our indoor environments.
Why Do I Need To Know Anything
About Mold?
Too much mold can affect the health of you and your
family. In addition, mold can damage or destroy building
materials such as the wood or gypsum board in our homes.
What Are The Health Effects of
Exposure To Mold?
Most people are not affected by exposure to mold,
unless they are exposed to a lot of mold. Unfortunately, we
are not quite sure what “a lot of mold” means. Furthermore
we don’t know if “a lot” of exposure to mold for “a brief time”
is worse than “not so much” exposure for a longer time.
We’re also not sure what “not so much” means. Each person
is different; what amounts to a “lot of exposure” for some
people is “not so much” for others. Remember, mold is
everywhere; we are all exposed to mold every day.
Exposure to mold can cause allergy in susceptible
people, but we don’t know how much exposure is necessary to
start the development of allergy. If you have asthma, exposure
to mold can cause an asthma attack or make your chronic
asthma get worse. At this point we do not know if exposure to
mold, especially early in life, can lead to the development of
asthma. Although exposure to “enough” mold can cause
allergy to mold in susceptible people, accurately diagnosing
the allergy can be difficult. Unfortunately, doctors can test for
allergy to only a very few of the thousands of species (kinds)
of molds that exist.
Only a few molds seem to be able to sometimes cause
an infection in healthy people; fortunately these molds do not
usually grow in buildings. However, people with a suppressed
immune system are much more susceptible to fungal (mold)
infections and many of these fungi do grow in wet buildings..
Individuals with AIDS, certain types of cancer and those with
organ (heart, kidney) transplants on certain drugs are much
more susceptible to fungal infections.
Molds sometimes make powerful chemicals called
mycotoxins. We think that molds make these mycotoxins to
decrease the growth of other molds and bacteria. Penicillin is
a mycotoxin that we use because it can kill certain bacteria.
Unfortunately some of these mycotoxins make people sick.
Mycotoxins can cause illness when they are inhaled, absorbed
through skin or ingested (swallowed). Presently we do not
know all that much about the health effects of most mycotoxins on humans. Most of what we know about mycotoxins
comes from exposure of farm animals to moldy grain or hay.
We do not have any tests that can determine whether mycotoxins are the cause of someone’s illness. We cannot easily or
reliably measure the level of mycotoxins in air samples to
determine exposure levels.
© 2002 Building Science Corporation
RR-0208: What You Need to Know About Mold
Page 1
How Can I Prevent Mold From
Growing In My Home?
The answer is simple: keep your home dry. If mold
does not have moisture it cannot grow. Remember mold
spores are everywhere so you cannot completely keep them
out of your house. Since our houses are built with wood
products and paper faced drywall, food for mold is always
present. Mold can also thrive on dust, cockroach and dust
mite feces, skin flakes and food particles. Oxygen is available
in the air so we cannot prevent mold growth by eliminating
oxygen. Therefore, controlling moisture is the only effective
strategy for preventing mold growth in our homes.
How Do I Keep My House Dry?
First, prevent excessive moisture levels in the air in the
house by using the exhaust fans in bathrooms and kitchens
where moisture is often generated. Make sure the air from
your clothes dryer exhausts from the house. Avoid using a
humidifier. If you think that you need a humidifier, first
measure the relative humidity inside your home. A device
called a hygrometer can measure the relative humidity. Try
to keep the relative humidity (RH) below 60%. There is rarely
a reason to use a humidifier if the RH is above 25%. If you
feel you must use a humidifier, measure the relative humidity
in the area receiving the humidified air. Never let the
humidifier raise the relative humidity above 35%.
Second, look for areas of dampness or wet spots.
Roofs, windows, basement walls and plumbing pipes sometimes leak. If a leak happens suddenly, dry the wet materials
as quickly as possible. Mold spores begin to grow 24-48
hours after a water leak. Dry the house quickly and mold will
not be a problem.
Small recurring or continuous leaks that are hidden in
walls, ceilings or floors present a different challenge since
mold growth could be extensive before it is detected. Be alert
for dampness in areas such as under sinks, in bathrooms and
in ceilings below bathrooms or other plumbing. If you notice
dampness in one of these areas or if you notice a strong musty
smell in one of these areas, contact your builder.
How Do I Get Rid of Mold In My
House?
That depends on how much mold is present and where
it is located. If there is only a small amount of mold, such as
in a bathroom or kitchen, scrub the moldy area with soapy
water, rinse thoroughly and allow to dry. If you have asthma
or severe allergies or a weakened immune system, get
someone else to do the clean-up.
If you have mold on a wall, floor or ceiling, the first
question is “where did the water come from?” Remember,
mold has to have water to grow. There is no point trying to
remove the mold if you do not also correct the moisture
problem that led to the mold. If the moisture problem
remains, mold will quickly grow back after the initial cleanup.
If mold is present in just one corner of a closet scrub it with
soapy water, rinse and thoroughly dry. Don’t store boxes right
up against that corner. Do something to keep that corner
warmer such as leaving a light on in the closet.
If you have mold growing under a window that leaked
or somewhere else that probably is due to a leak, you should
have someone with experience dealing with mold in buildings
evaluate the building. Why? Again, you need to fix the
underlying moisture problem. And more importantly you can
release a lot of mold from inside a wall cavity if you don’t do
the work properly. You can turn a “little bit” of exposure into
a “lot of” exposure very quickly.
What Should I Do If I Suspect I
Have Mold In My House?
First and foremost, do not panic. Remember mold is
everywhere. Even if you do have more than “normal”
amounts of mold in your house, you have not necessarily been
exposed to it. Exposure means that the mold or mold spores
or toxins have gotten inside your body by inhalation (breathing), ingestion (eating) or absorption through your skin. If
mold is inside a wall in your home but it does not get into the
air, you have not been exposed. The mold has to get from the
wall cavity into the air in the house before you can be exposed
to it. However, that does not mean that it’s good to ignore the
mold if you know you have it somewhere in your house. The
mold will continue to grow (as long as it has water), eventually damage the building and increase the chances that you
will be exposed.
© 2002 Building Science Corporation
RR-0208: What You Need to Know About Mold
Page 2
What You Need to Know About Mold
About the Authors
Joseph Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Massachusetts. Joe is an ASHRAE Fellow and an internationally recognized
authority on indoor air quality, moisture, and condensation in buildings. More
information about Joseph Lstiburek can be found at
www.buildingscienceconsulting.com.
Terry Brennan, MS, is a building scientist and educator with Camroden Associates,
Westmoreland, New York.
Nathan Yost, MD, investigated mold and moisture issues for Building Science
Corporation.
Direct all correspondence to: Building Science Corporation, 30 Forest Street,
Somerville, MA 02143.
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